Gas sensors having field-effect transistors are generally known. Their basic measuring principle is based on a diffusion of the gases to be detected into a gas-sensitive layer, which induces a change in the potential at the FET. A field-effect transistor having a source (S), drain (D) and gate (G) region is situated on a substrate, a gas-sensitive layer being applied on the gate (G) region. If the FET-based gas sensor (FET gas sensor) is exposed to a gas or a gas mixture, the gas or gas mixture to be detected diffuses into the gas-sensitive layer and in so doing, produces or changes the potential at the boundary surface to the channel region (C) between the source (S) and the drain (D) regions. This change in potential is able to be tapped as a measuring signal via a current or voltage change. Various modifications or further developments based on this basic principle are known as well.
The sensitive layer is usually separated from the gate (G) region by an intermediate layer disposed in-between. On the one hand, this protects the gate (G) region from harmful gas actions. On the other hand, the sensitive layer, which usually is electrically conductive in order to allow for the adjustment of a defined potential at the gate (G) region, is electrically insulated from the channel region (C) of the FET. That is to say, the intermediate layer is used as a protective layer for the gate (G) region and simultaneously as an insulation layer.
In German Patent Application No. DE 10 2005 008 051, for example, a gas-sensitive FET and a method for its operation are described, the sensitive layer having a porous design with open pores and any desired thickness and being directly applied on an insulation of a FET while avoiding an air gap. The sensitive layer is situated directly above the channel region of the FET or cooperates indirectly with the FET via a potential-free floating gate electrode.
In order to be able to perform the function of a chemical protective and electric insulation layer in a reliable manner, the separation layer is formed by a chemically inert, electrically insulating, closed and compact layer, if possible. The usual materials for this purpose are, for example, thermal oxides, LPCVD (low pressure chemical vapor deposition) oxides or also LPCVD nitrides. These layers have little surface roughness. This results in modest adhesion of the sensitive layer to the separation layer.
On the other hand, excellent adhesion of the sensitive layer to the FET is an important prerequisite for a reliable and long-term functionality of the gas sensor. For the sensors are often used in a measuring environment that exposes the sensitive layers to high mechanical or chemical stresses. One pertinent example is the use of a gas sensor for the NOx detection in the exhaust tract of an internal combustion engine. Mechanical stressing may be caused by, for example, strong vibrations, a very dynamic gas flow, or also by temperature-related freezing. Chemical stressing may be caused by exhaust gas or moisture in the measuring environment, for instance.
Consequently, there is demand for a gas sensor having a field-effect transistor in which the sensitive layer has improved adhesion to the gate (G) region of the sensor compared to current FET gas sensors known from the related art.